Feature request: distinguish between manual removal and natural LRU eviction

[jamesmoessis]

Wondering if you would accept a PR for the following:

My use case is that I don't want the eviction callback to be called when I am manually calling c.Remove(k), only when natural LRU eviction occurs. Reasoning: I have a metric of how long something stays on the LRU cache. Then if I manually Remove() from the cache, this metric is polluted because now it doesn't track the natural LRU eviction time, but just the time since insertion of the key.

Would you accept a PR to add a new method to the simple cache, for example a Delete() or RemoveWithoutEvictCB() method. I would be happy to contribute, cheers!